Satellite broadcasting systems for transmitting programming content have become increasingly popular in many parts of the world. Direct Broadcasting Satellite (DBS) systems transmit television programming content, for example, to a geo-synchronous satellite, which broadcasts the content back to the customers. In such a wireless broadcast environment, the transmitted programming can be received by anyone with an appropriate receiver, such as an antenna or a satellite dish.
In addition, a number of satellite broadcasting systems have been proposed or suggested for broadcasting audio programming content from geo-synchronous satellites to customers in a large coverage area, such as the continental United States. Satellite broadcasting systems for television and radio content provide potentially national coverage areas, and thus improve over conventional terrestrial television stations and AM/FM radio stations that provide only regional coverage.
Code Division Multiple Access techniques have been proposed for satellite broadcasting systems to permit a number of programming channels to be transmitted on the same carrier frequency. Code Division Multiple Access techniques transmit multiple information signals on the same carrier frequency, and differentiate each programming channel by encoding the channel with a unique orthogonal code.
CD Radio Incorporated has proposed a satellite broadcasting system having two satellites and a group of repeaters to provide audio service. The CD Radio system is described, for example, in U.S. Pat. Nos. 5,278,863, 5,319,673, 5,485,485 and 5,592,471. In a developed area, the direct line of sight (LOS) between a mobile receiver and the transmitters on the satellites and repeaters can be blocked, for example, by underpasses or other structures. Thus, many satellite broadcasting systems transmit a delayed version of each program channel with the on-time version of the program channel to permit uninterrupted reception in the event of a blockage. U.S. patent application Ser. No. 09/220,722, filed Dec. 24, 1998, a parent application to the present patent application, discloses a code division multiplex (CDM) satellite broadcasting systems that transmits both the on-time and delayed versions of each program channel transmitted from the same satellite.
In digital broadcasting applications, feedback or retransmission of lost symbols or packets are not desirable solutions, because of the nature of the broadcasting environment. Thus, forward error correction schemes, such as convolutional and block coding, have been used to combat fading or packet loss in a broadcast environment. Forward error correction schemes protect the transmitted signals so that even if part of a signal is received with error, the error may be corrected or minimized using other transmitted information that is received under more favorable conditions. Convolutional codes effectively reduce reception errors due to relatively slow fading, provided appropriate interleaving is also employed. Interleaving scrambles a signal over a certain time interval. In order for convolutional codes to work properly, consecutive symbols presented to a signal channel decoder at the receiver should be uncorrelated. For example, the consecutive symbols should be separated timewise from one another during transmission, such as occurs by interleaving.
If block-coded symbols are interleaved over the duration of many blocks before transmission, symbols associated with a lost packet will be de-interleaved by the receiver and found among many different coded blocks. Thus, the number of symbol errors that may occur in each coded block is reduced, and the likelihood that a selected block code will correct all symbol errors in a transmitted signal is correspondingly increased. A large interleaver length, however, results in large time delays when first tuning the receiver to a broadcast program, since decoding and reproduction of the original program content can start only after an entire interleaved packet has been buffered or stored at the receiver.
While a delay of as much as several seconds between signal reception and program reproduction at the receiver may go unnoticed by a user as long as the receiver remains tuned to one program, such delay can be intolerable when first tuning the receiver to select a program, or while re-tuning to select a different program. Then, delays of at most a fraction of a second may be tolerable. Thus, while large interleaver lengths help to ensure that a program source signal is transmitted, received, decoded and reproduced with minimum error due to fading and noise, the interleaver lengths must be relatively short if program decoding delays at the receiver are to be minimized while the receiver is being tuned or switched among different programs. U.S. patent application Ser. No. 09/203,663, filed Dec. 2, 1998, a parent application to the present patent application, discloses a technique that allows a receiver to reproduce a program source with minimal delay when the receiver is first tuned to the program source. Generally, a tuning channel is provided for each program channel that utilizes a shorter interleaver length than the corresponding primary program channel.